I wouldn't consider deleting profiles because they're inactive for a certain amount of time - but I would consider making them "inactive", so they don't appear in listings until they become active again.
A higher level of importance will soon be placed on the Response Rate, and those with a very low response rate will feel the effects of having a low one, and those who have a higher RR and a higher level of interaction with the site will reap the benefits. I'll go into more detail in a week or 2 once it starts to get rolled out
James